Hi to all, read many things on here now have joined.79' F-150 4 speed 300 stright six 4x4 and its a step side. Now here is the issue. I have zero oil pressure at start for a 10th of a sec then it hits about 40 psi to 50psi as I drive or it warms up the pressure will go to zero as the idle goes down. Now the gauge I think is ok cause when it hits zero I get a knocking sound if I bring the rpms up to 2,000rpm the oil gauge will go to 10psi & knock will go away. I love this truck have put a lot into it cause the guy I bought it from didn't know anything still haven't found what he did to turn signals emergency flashers works but missing relay to left side & the plug, wires change color every where lots or tape & wire nuts. So I got 2 push buttons drilled into side of stearing wheel housing so I push with my Fingure to make left or right side work. So not sure of what has been done but I'm not helpless to fix stuff. So anybody that could help me b4 I have to hand it over to ASE person I would be forever greatful to you
THANKS

 

How long since oil change and what oil is in it?

 

To be honest, more than likely it's time to rebuild the bottom end.
What weight oil are you running?
If the bottom end is worn, you can buy a little time by running heavy weight oil and installing a high volume (volume, not pressure) oil pump.
I had a 360 that I ran for years with straight SAE 50 weight and two cans of STP. Even with a high volume oil pump, the best it could do was around 10-15PSI when warm. On teardown, the bearings were shot but it didn't knock. The crank journals were worn but smooth.

 

IMO suggesting a rebuild or a straight 50 is jumping the gun. I've had a few engines show symptoms like this. Three simple facts, oil gets thinner when hot, oil gets thinner over time, and tolerances get larger over time.

Nobody will say that a straight 5 weight oil should be run, but if you put in a 5w30 and leave it in there long enough, that's what you got.

It's summer and no reason to suspect the OP has an oil cooler. Oil gets very thin very fast when overheated.

So if he hasn't already, toss some fresh 10W40 and a filter in it, see what happens, and go from there.

 

A lot can be learned by simply looking, take your oil cap off and shine a light in there, is it clean or full of gunk? How many miles on the engine? Are you using a quality oil filter and oil?
These 300's can take a lot of abuse but they do wear out

 

Change oil b4 it was due full synthetic 10w 30

 

I use frame oil filter & castrol full synthetic 10w30. So looking at crank bearings. I'll take the oil cap off in morning to look in with flash light

 

Time for a 10W40 then, my 460 does this with 10/5W30, changed to a 5W40 and no problems.

 

I would drop the Fram filter and go with Motorcraft, Wix or NAPA gold.
I have a personal dislike for Fram that goes back to the 70's.

 

Me no like the "Orange can of Death".

That said, it could be a bad filter, or the oil pickup screen could be clogged up. heck, it's even possible the oil pickup tube is cracked/loose......

 

In this instance I'm all for the fram, they aren't the best filters but they are the most free flowing.

As for the pickup, I'd think that a possibility except that the OP said the pressure rises quickly with RPM and on startup. A pickup issue would be more constant.

 

like others have suggested, change the oil, use 10w40, 15w40 or 20w50...and get a good oil filter-meaning not FRAM....Ford engines and FRAM oil filters do not like eachother..

Did this problem start after you switched your 300 to full synthetic? chances are the full synthetic loosened up some crap in the engine and plugged something..I have heard somewhere that it's not good to switch over a high mileage, dirty inside, engine to full synthetic, because the synthetic does what it's supposed to do and "cleans" the engine, loosening up the gunk and chunks..

 

Thanks for all the great ideas I've heard you can not put real oil back in after you go synthetic does anybody know if that's true it didn't start after I did the 1st change but did start b4 2nd one I don't drive the truck alot more the not I'm just starting it for a few minutes to keep battery up & re oil the motor. Once again the guy I bought it from have heard from people who know him that he is a cheat & lies all the time but I've had the truck for 3 years now

 

You can switch back and forth between regular and synthetic oil without worry. You can even use regular oil to top up synthetic between changes.
Heck, they even sell oil that is part regular and part synthetic, AKA synthetic blend.

Switching a high mileage engine to synthetic can be risky, as mentioned. This stuff can loosen the built-up crud inside, and cause the oil pickup screen to get clogged, or the gaskets/seals to start leaking quite badly.


As for the fram filter. Many have heard rumors about engines being killed by these things. I've seen it first hand. It looked like toilet paper thru the entire oiling system.....

 

My dislike for Fram filters is also from first hand experience, not rumors.
Back in the early 70's the company I was working for had a piece of equipment that was run by a very expensive Sundstrand hydrostatic transmission.
We did the routine, scheduled maintenance on it which at the service interval required a hydraulic oil and filter change. We used 45 gallons of Dexron transmission fluid and a new Fram filter. With less than 10 hours on the oil change the machine malfunctioned. Long story short the Sundstarnd pump had failed. Why? The Fram filter had disintegrated and dumped a lot of debris into the system, trashing the pump.
Bottom line, it cost over $6,000.00 (in 1970's dollars) to repair. Fram's response? Not our problem.
So yeah, I don't like Fram.